1. Field of the Invention
This invention relates to a process for separating nickel, cobalt and/or chromium from iron in metallurgical products, e.g. ferronickel, ferrocobalt and ferrochromium, containing these metals.
2. Description of the Prior Art
Several metals are refined electrolytically by using insoluble anodes; a raw material containing the metal, usually a roasted product, is dissolved in the acid produced in the electrolysis, and the solution obtained is purified chemically by a separate process before it is fed back into the electrolysis. It is known that zinc can be so refined.
Pure nickel is produced hydrometallurgically by the use of insoluble anodes; by using soluble, so-called sulfide matte anodes; or by dissolving this sulfide matte chemically and purifying it into an electrolyte.
The latter case must be considered the present state of the art to which production processes have been developed.
The process for dissolving sulfidic raw material which contains nickel is not applicable as such to the dissolution of ferronickel because of the great amount of iron dissolved from it in the solution, the difficult treatment of the precipitate so produced, and the slowness of its dissolution. A dissolution and purification process based on sulfates was desired for ferronickel, in particular, a process from which the above disadvantages had been eliminated.
There are difficulties which often occur in the hydrometallurgical treatment of products with high iron contents such as ferronickel, ferrocobalt and ferrochromium; precipitation of the iron, filtering the obtained ferrihydroxide precipitate, and above all washing the precipitate sufficiently clean of the mother liquor.
Too much of the valuable metal content of the mother liquor is removed with the precipitate. Furthermore, it is very difficult to remove from the electrolytic cell the anode sludge produced in the electrolytic dissolution of ferronickel and other iron-containing materials. Attempts have been made to develop processes for the treatment of ferronickel, and one such process is introduced in Finnish Pat. No. 44,300, in which ferronickel is dissolved electrolytically as an anode, and the impure nickel solution thereby produced is purified chemically by a separate process and the obtained pure nickel-containing solution is pumped into the cathode chamber of the electrolysis, where the nickel is precipitated onto the cathode with the help of electricity.
The solution used is a chloride-based electrolyte which is said to have a good anodic corrosive effect but which is even otherwise highly corrosive (muriatic acid+ chloride) to the entire apparatus and the rest of the surroundings. Poisonous chlorine gas easily develops during the electrolysis which must be used for the chemical purification of the anolyte and for the removal of both iron and cobalt.